Water Transport in Plant
The water absorbed by the root hairs is translocated upwards through the xylem. The mystery of the upward movement of water is yet to be solved in a satisfactory way. The upward transport of water in plants which are 400 feet high has not been satisfactorily explained till date.
Though the mechanism for upward movement of water or ascent of sap is not clear, it has been proved that ascent of sap takes place through xylem.
The girdling experiment was done is a plant with thick stem where the outer layer of phloem was removed. Still it was found that ascent of sap continued to take place proving that ascent of sap takes place through the xylem.
A young tomato or balsam seedling was taken and kept in a beaker containing water coloured with eosin. After sometime it was seen that streaks of red colour were running up the stem. When a cross section of the stem was taken, it was found that xylem was coloured proving that ascent of sap takes place through the xylem tissue.
A number of theories have been put forward at various times to explain the mechanism of ascent of sap. These are (i) Vital theories, (ii) Root Pressure theory and (iii) Transpiration pull.
These theories had been given very early and have only historical importance. Godslewski gave the relay pump theory. According to this theory the pumping of water takes place upwards due to the vital activities of xylem parenchyma and xylem rays.
J.C. Bose has put forward the pulsation theory. According to this theory water is pumped up due to the contraction and expansion of innermost cortical cells which creates a pulsation causing upward movement of water.
This is also called as cohesion-tension theory put forward by Dixon and Joly (1894) and supported by Renner, Curtis andClark. This theory is based on a number of features.
Mutual attraction between water molecules is called cohesion and this force may have a value as high as 350 atmospheres.
The wall of the tracheids and vessels which transport water are made up of lignin and cellulose and have high affinity for water and this is called adhesion.
Xylem vessels have perforated end walls and form a tubular structure from roots to the shoot tip. This provides a continuous channel for movement of water which cannot be pulled away from xylem wall due to cohesive and adhesive properties.
The transpiration taking place through leaves causes negative pressure or tension in xylem sap which is transmitted to the root. This is called transpiration pull which is responsible for the movement of water column upward.